char *strtok( char *str1, const char *str2 )

- prototype in string.h
- parses str1 for tokens and separators, where separators are
defined in str2
- first call returns a pointer to the first character of the first
token in str1 and writes a null character into str1 immediately
following the returned token; subsequent calls with NULL for
the first argument will work through the string str1 in this way
until no tokens remain
- when tokens are exhausted, returns NULL